1. Field of the Invention
The present invention relates generally to a redundancy arrangement for a circuit using a focused ion beam and more particularly pertains to a static redundancy arrangement for a circuit using a focused ion beam anti-fuse methodology which reduces the circuit layout area compared to a conventional dynamic redundancy arrangement. Furthermore, the static redundancy replacement significantly reduces the switching activity compared to a prior art dynamic redundancy scheme, resulting in less power, a simpler design and higher speed.
2. Discussion of the Prior Art
The semiconductor industry has used focused ion beam writing techniques for many applications, including changing the conductivity of insulating films, depositing dielectric or conductive materials, and trimming of circuits. As reported by M. F. Edinger et al. [Focused Ion Beam Writing Of Electrical Connections Into Platinum Oxide Film, Appl. Phys. Let. (USA) Vol. 76, No. 23, Jun. 5, 2000, P3445-7], a focused Ga +ion beam system has been demonstrated to change the sheet resistance of an insulating platinum oxide film from 4E9 ohm/square into a conducting film with a sheet resistance of 5E2 ohm/square. The large decrease in resistance is caused by an oxygen loss caused by the focused ion beam irradiation. It has been reported that the resolution of the focused ion beam patterning is more than one order of magnitude higher than the resolution of patterning by a laser. In addition, the film quality after ion irradiation is more homogeneous than the film quality after laser irradiation.
The focused ion beam systems are very attractive and versatile tools to make precision modifications in the deep submicron range. For example, using a liquid metal ion source of Ga+ions, the ion beam can be focused down to 5 nm, allowing chemical processes to be confined to nanometer dimensions. The same tool makes it feasible to deposit metals and insulators by sputtering processes, and also to conduct chemical assisted etching and doping in confined areas. The focused ion beam tools have also been used commonly for rapid and flexible chip modification [J. Melingailis, J. Vac Sci. Technol, B5, 469, (1987); T. Tao, W. Wilinkinson, J. Melingailis, J. Vac. Sci. Technol. B9, 162, (1991)].
In focused ion beam metal deposition processes, a stream of precursor gas is directed towards the area of interest where the focused ion beam is writing (the local area is in the mTorr pressure range while the chamber base pressure is in the low 10-7 Torr pressure range). Incident ions of Ga, Cs O2, Ar, N2 etc. break up the gas molecules that are adsorbed onto the surface leading to a metallic deposit. Commonly used metal precursor gases are organometallic or metal halides. Depositions of Au, W, Ta, Al, Pt have been demonstrated. To lower the sheet resistance of the deposits, a post deposition anneal can be carried out.
Focused ion beam deposition of silicon dioxide from tetramethoxysilane and oxygen using a Si ion source has also been reported. Typical beam acceleration voltages and currents are 5 to 50 keV and 0.1 to 2 nA respectively.
Focused ion beam assisted deposition has also been used for circuit modification in integrated circuits [Wang Tai-Ho, U.S. Pat. No. 5,741,727, Circuit Modification And Repair Using A Low Resistance Conducting Metal Bridge And A Focused Ion Beam].
Focused ion beam milling can also be performed with a pure positively charged Ga ion beam, which is usually generated by applying a high electric field between the liquidmetal ion source (Ga) and an ion extractor. The beam energy is typically around 30 to 50 keV and the beam current is typically from 1 to several tens of nA. The beam size resolution can be down to a few nanometers. The beam is programmed to raster across the wafer surface, which is maintained under a high vacuum (around 10-7 mbar).
Nanometer-size GaN/AIGaN device structures fabricated by focused ion beams have also been reported [Kuball, M. Benyoucef, M. Morrissey, F. H. Foxon, xe2x80x9cFocused Ion Beam Etching Of Nanometer-Size GaN/AIGaN Device Structures And Their Optical Characterization By Micro-Photoluminescence/Raman Mappingxe2x80x9d, Materials Research Society Symposium-Proceedings, V595, 2000, Materials Research Society, Warrendale, Pa., USA, p W12.3.1-W12.3.6].
To enhance the etching rate, and to minimize the Ga stain, additional gases can be flowed to the area of interest. For example, xenon difluoride can be used to enhance the etching of silicon dioxide, halogen gases have been used to enhance the etching of aluminum, and water vapor has been used to assist in the removal of carbon-based materials.
However none of the techniques mentioned above have ever been implemented in semiconductor fuse and anti-fuse applications, or used to conduct massive programmability in an automatic manner.
Accordingly, it is a primary object of the present invention to provide a redundancy arrangement for a circuit using a focused ion beam which is particularly applicable to pitch-limited circuitry, particularly a focused ion beam antifuse approach which provides advantages of a simple design which can be provided in a smaller area as it doesn""t require address comparison, column decoding, and a fuse bank, and uses less power by reducing switching activity compared to a prior art dynamic redundancy scheme.